Throughout maritime history navigators have sought an accurate reliable method of determining their position on the surface of the earth and many instruments such as the sextant were devised. During the second world war, a long range radio-navigation system, LORAN-A, was developed and was implemented under the auspices of the United States Coast Guard to fulfill wartime operational needs. At the end of the war there were seventy LORAN-A transmitting stations in existence and all commercial ships, having been equipped with LORAN-A receivers for wartime service, continued to use this navigational system. This navigational system served its purpose but shortcomings therein were overcome by a new navigational system called LORAN-C.
Presently, there are eight LORAN-C multi-station transmitting chains in operation by 1980. This new navigational system will result in an eventual phase-out of the earlier LORAN-A navigational system.
LORAN-C is a pulsed low-frequency (100 kilohertz), hyperbolic radio navigation system. LORAN-C radio navigation systems employ three or more synchronized ground stations that each transmit radio pulse chains having, at their respective start of transmissions, a fixed time relation to each other. The first station to transmit is referred to as the master station while the other stations are referred to as the secondary stations. The pulse chains are radiated to receiving equipment that is generally located on aircraft or ships whose position is to be accurately determined. The pulse chains transmitted by each of the master and secondary stations is a series of pulses, each pulse having an exact envelope shape, each pulse chain transmitted at a constant precise repetition rate, and each pulse separated in time from a subsequent pulse by a precise fixed time interval. In addition, the secondary station pulse chain transmissions are delayed a sufficient amount of time after the master station pulse train transmissions to assure that their time of arrival at receiving equipment anywhere within the operational area of the particular LORAN-C system will follow receipt of the pulse chain from the master station.
Since the series of pulses transmitted by the master and secondary stations is in the form of pulses of electromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses from a master and a secondary station represents the difference in the length of the transmission paths from these stations to the LORAN-C receiving equipment.
The focus of all points on a LORAN-C chart representing a constant difference in distance from a master and a secondary station, and indicated by a fixed time difference of arrival of their 100 kilohertz carrier pulse chains, described a hyperbola. The LORAN-C navigation system makes it possible for a navigator to exploit this hyperbolic relationship and precisely determine his position using a LORAN-C chart. By using a moderately low frequency such as 100 kilohertz, which is characterized by low attenuation, and by measuring the time difference between the reception of the signals from master and secondary stations, the modern-day LORAN-C system provides equipment position location accuracy within two hundred feet and with a repeatability of within fifty feet.
The theory and operation of the LORAN-C radio navigation system is described in greater detail in an article by W. P. Frantz, W. Dean, and R. L. Frank entitled "A Precision Multi-Purpose Radion Navigation System," 1957 I.R.E. Convention Record, Part 8, page 79. The theory and operation of the LORAN-C radio navigation system is also described in a pamphlet put out by the Department of Transportation, U.S. Coast Guard, Number CG-462, dated August, 1974, and entitled "LORAN-C User Handbook".
The LORAN-C system of the type described in the aforementioned article and pamphlet and employed at the present time, is a pulse type system, the energy of which is radiated by the master station and by each secondary station in the form of pulse trains which include a number of precisely shaped and timed bursts of radio frequency energy as priorly mentioned. All secondary stations each radiate pulse chains of eight discrete time-spaced pulses, and all master stations transmit the same eight discrete time-spaced pulses but also transmit an identifying ninth pulse which is accurately spaced from the first eight pulses. Each pulse of the pulse chains transmitted by the master and secondary stations has a 100 kilohertz carrier frequency, so that it may be distinguished from the much higher frequency carrier used in the predecessor LORAN-A system.
The discrete pulses radiated by each master and each secondary LORAN-C transmitter are characterized by an extremely precise spacing of 1,000 microseconds between adjacent pulses. Any given point on the precisely shaped envelope of each pulse is also separated by exactly 1,000 microseconds from the corresponding point on the envelope of a preceding or subsequent pulse within the eight pulse chains pulses. To insure such precise time accuracy, each master and secondary station transmitter is controlled by a cesium frequency standard clock and the clocks of master and secondary stations are synchronized with each other.
As mentioned previously, LORAN-C receiving equipment is utilized to measure the time difference of arrival of the series of pulses from a master station and the series of pulses from a selected secondary station, both stations being within a given LORAN-C chain. This time difference of arrival measurement is utilized with special maps having time difference of arrival hyperbola information printed thereon. These maps are standard LORAN-C hydrographic charts prepared by the U.S. Coast Guard and the hyperbola curves printed thereon for each secondary station are marked with time difference of arrival information. Thus, the difference in time arrival between series of pulses received from a master station and selected ones of the associated secondary stations must be accurately measured to enable the navigator to locate the hyperbola on the chart representing the time difference measured. By using the time difference of arrival information between a master station and two or more secondary stations, two or PG,4 more corresponding hyperbolae can be located on the chart and their common point of intersection accurately identifies the position of the Loran-C receiver. It is clear that any inaccuracies in measuring time difference of arrival of signals from master and secondary transmitting stations results in position determination errors. This requires that oscillators internal to the Loran-C receiver be calibrated frequently in order to avoid measurement errors caused by oscillator inaccuracy.
There are other hyperbolic navigation systems in operation around the world similar to Loran-C, and with which my novel receiver can readily be adapted to operate by one skilled in the art. There is a Loran-D system utilized by the military forces of the United States, as well as the aforementioned Loran-A system. Others are DECCA, DELRAC, OMEGA, CYTAC, GEE and the French radio WEB, all of which operate in various portions of the radio frequency spectrum and provide varying degrees of positional accuracy.
Loran-C receiving equipment presently in use is relatively large in size, heavy, requires relatively expensive oven controlled crystal oscillators, requires frequent calibration, and requires relatively large amounts of power. In addition, present Loran-C receivers are relatively expensive and, accordingly, are found only on larger ships and aircraft. Due to the cost size, weight, and power requirements of present Loran-C receiving equipment, such equipment is not in general use on small aircraft, fishing boats and pleasure boats. In addition, Loran-C receiving equipment presently in use required anywhere from five to ten minutes to warm up and provide time difference measurement information.
The signals presently received by LORAN-C navigation receivers have very low signals to noise ratios and it is difficult to locate the third cycle positive zero crossing conventionally used in making the time difference measurements between signals received from the master and secondary stations. This problem is exacerbated by noise generated within the circuitry of LORAN-C navigation receivers and particularly in the front end circuitry in the signal path immediately following the receiver antenna.
Thus there is a need in the art for improved circuitry and techniques to minimize the noise internally generated or to minimize the effect of noise generated internal to LORAN-C receivers. It is a feature of this invention to minimize the effect of noise generated internally to a receiver by averaging out the noise.
There is also a need in the art for inexpensive oscillators within LORAN-C receivers that never require calibration yet the operation of the receivers is as if the oscillators are as accurate as a laboratory standard oscillator. Such oscillators increase the accuracy and reliability of navigation information output from the receiver.